Why should you only use MESA/Boogie® tubes in MESA amps?

Posted by Randall Smith

MESA/Boogie’s business is designing and building high performance amplifiers. For this, tubes with a variance within a narrow range are required. We have a very sophisticated computer based tube testing system (nicknamed "Robotube") that matches and measures tubes over seven important parameters and technicians who add a few tests that only the human ear can measure. This insures we end up with only the tubes that meet our stringent demands.

Tubes don’t "draw" more or less bias. The way a bias supply is connected to a tube is akin to a dead end road, it just trails off to nowhere without really completing a circuit. It's a static voltage and regardless of what tube is in the socket or even if the tubes aren't plugged in at all, it doesn't change the bias voltage a bit. Since a bias supply needs to put out the right voltage and never vary, MESA amps are hard wired to the correct values and never need adjustment. And for more than 40 years, that's how MESA/Boogies have been built.

Time to change tubes? Just plug our tubes into any one of our amps and you're done. No tech needed. No bills and no worry about biasing. We only sell the tubes that meet the specifications required for our amplifiers.

Want to go a bit deeper?

You might be thinking, "But I thought I just read that tubes don't "draw" bias, therefore they don't affect the bias supply and thus it doesn't need to be adjustable." That's right. Tubes don't affect the bias setting, but the bias setting does affect how the tubes work. How it affects the tubes is difficult to measure.

When you set the bias (whether it's by selecting the right resistors, as we do, or adjusting a trimmer - which is quicker) what you are doing is establishing the correct amount of idle current that flows through the power tubes. But you can't adjust the current directly, you can only change it by adjusting the amount of bias voltage that goes onto the tubes' control grids.

Voltage and current are not the same. Current is the amount of electricity (the "quantity") and is measured in amperes. Voltage is the degree of electric charge. Current and voltage are two totally separate electrical parameters, though when you multiply them together, you get power (which is measured in watts).

When you set the bias of an amplifier, you are adjusting the static voltage at the control grid of the tube in order to produce a desired amount of idle current flowing to the tube's plate. A small change in grid voltage, produces a large change in the amount of current flowing. That's basically how a tube works and the essence of amplification: a small change causing a large change. Here, it's a small voltage change causing a large current change.

The bias conditions are what determines how much current flows through the power tubes when you're not playing. What drives your speakers is fluctuations in that current flow when you are playing. If the amount of current increases and decreases 440 times per second, then you'll hear an A note. If the fluctuations in current flow are large and still at 440 per second, you'll hear an A that is loud!

For purposes of biasing, it's the amount of "plate current" flowing with no signal applied that's important. Unfortunately, current is hard to measure because the circuit must be interrupted (as in "cut the wire") and the meter spliced "in series" with the broken circuit. But measuring voltage is easy. It is not necessary to interrupt the circuit because a voltage reading can be taken in parallel with the circuit intact. Thus, as a matter of convenience, most bias settings are given in volts at the grid, even though current through the plate is the important factor. As long as the tubes are "in spec", the right bias voltage will always give the correct plate current, so there's no need for the bias voltage to be adjustable!

Merely plug a matched set of MESA tubes into one of our amps and you're ready for tone. Guaranteed. How much money and trouble that has saved MESA/Boogie players couldn't be estimated.

MESA’s rigorously tested, hand selected tubes are warrantied for six months. We always have a great selection and are happy to assist you with getting the right tubes for your amp!

This article was originally written by Mesa Founder, Designer & President - Randall Smith - and published in September of 1996. It has been edited for this post.